This invention relates to a sheet width determining apparatus and to a width determining apparatus used with a thickness gauging method and system with automatic calibration for sheet width variation. More particularly, the invention relates to methods and systems in which the edges of a sheet in transit past a gauging system are regularly and systematically located by the gauging system, to establish limits of gauge movement transverse to the direction of movement of the sheet and to enable resetting of measurement locations across the sheet, or to measure the width of the sheet being extruded.
Methods and systems for measuring the thickness of sheet are known in the art such that a gauge traverses the moving sheet to measure sheet thickness as the sheet moves past the thickness measurement station. The gauge either continuously measures thickness as it moves across the sheet or it makes measurements only at predetermined locations across the sheet. One system described for use with a plastic sheet extruder is set forth in the inventor's U.S. Pat. No. 4,734,922 entitled Nuclear Gauge Traverse, which is assigned to the assignee of the present invention. The known systems typically use a nuclear gauge to determine the profile of thickness across the width of the plastic sheet being extruded. The nuclear thickness gauge is either a gamma backscatter type or a Beta gauge. It is placed on a travelling mount, which moves the gauge back and forth across the width of the sheet. Backscatter gauges are located entirely on one side of the sheet. Beta gauges are usually of the transmission type, whereby the source is located on one side of the sheet, and a receiver is located on the other side. In either case, the signal arriving at the receiver is a measure of the basis weight, or mass per unit area, of the sheet at the point where measurement is occurring. Since the density is known, this measurement can be used as a measure of the thickness of the sheet. Electronics associated with the gauge give an output indication of the thickness in commercially available gauges.
Conventionally, the traverse, which is to say the travelling mount, moves in one direction across the sheet, until it strikes a limit switch. This switch reverses the drive motor, so that the traverse goes back across the sheet in the other direction, until it strikes a second limit switch. Again the drive motor reverses direction, and the gauge goes back and forth across the sheet in this manner. As described in the inventor's above-mentioned patent, the sheet thickness information derived by these thickness gauges is used to make corrections at the die, where the sheet emerges from the extruder. Die bolt adjustments, whereby die bolts regularly spaced along the die opening are selectively adjusted or heated, to alter the opening size in the sheet thickness direction, and die heaters that vary the temperature of the die lips to control the thickness of the emerging sheet by altering viscosity, are two means by which sheet thickness measurement feedback information corrects the sheet thickness profile across the width of the sheet.
In the traverse of the inventor's aforementioned patent, the speed with which the thickness profile of the sheet can be established is improved. Whereas previously, the slow response time of nuclear gauges required a slow movement of the gauge across the sheet, the inventor reduced the time necessary to develop a profile of the thickness across the sheet by recognizing that the locations of interest were those locations across the sheet corresponding to the lateral spacing of the die bolts or die heaters that adjusted sheet thickness there. By rapidly moving from one measurement location to the next, pausing long enough to make a thickness measurement using the nuclear gauge, and then rapidly moving to the next location in "jackrabbit" fashion, the wasted time between measurements was held to a minimum. Most of the time spent during a traverse was spent at the locations where measuring the sheet thickness was to occur.
There remained, however, a further problem in that it was necessary to calibrate this or earlier traversing gauges to account for the width of the particular sheet being gauged. Because the sheet width of the sheet being extruded does not remain constant throughout the extrusion process, the system had to be calibrated anew from time to time to assure that the gauge was traversing nearly all of the sheet, but would not move beyond its edges. The need for calibration and recalibration is apparent from, for example, the problem of "neck-in" experienced during the extrusion of plastic sheet. A plastic sheet extruded through a die with a 30 inch wide slot, for example, will be 30 inches wide at the die, but there is a stretching effect between the lips of the die and the chill rolls that the plastic sheet typically is extruded onto. The sheet is stretched like a rubberband as it exits the die, and like a rubberband, when it stretches, it contracts in width. A 30 inch wide sheet will typically be 27-28 inches wide at the chill rolls. Sometimes the sheet is extruded downward into a water bath, and then the neck-in is even greater. A 76 inch wide sheet may be stretched down to 67 inches wide.
Calibration of a sheet gauging system should take into account the neck-in of the sheet and its effect on the location across the sheet of those places where thickness is directly affected by manipulation of correspondingly located die bolts or die heaters. The starting point for such calibration is location of the edges of the sheet. Moreover, because the sheet in certain extrusion processes varies in its width from time to time, for example when a water bath water level changes, calibration should be repeated throughout the extrusion process.
The width reduction also affects the positions of the test locations that were aligned with die bolts or heaters as the sheet emerged at the die. These test locations can be corrected manually, but it is a time consuming endeavor. Regularly manually resetting limits and gauging locations, then, is not an attractive prospect when economical operation and high rates of production are objectives.
Aside from the effect of sheet width variation on thickness gauging, width measurement at regular intervals can be used to determine that sheet width is within tolerance. Variations from desired width may call for corrective procedures to bring the width back to within acceptable tolerance.